Power supplies typically include rectifier circuits and transformers for generating a direct current (DC) output voltage from an alternating current (AC) input voltage. Typically, a first rectifier circuit is connected across a primary winding of the transformer, and a power metal oxide semiconductor (MOS) transistor is connected in series with the primary winding of the transformer for driving a current through the primary winding. The first rectifier circuit receives the AC input voltage and provides a rectified voltage to the primary winding of the transformer. A second rectifier circuit is preferably connected across a secondary winding of the transformer and provides the DC output voltage. The DC output voltage can be used to supply operating power to electronic devices such as computers, televisions, printers, battery chargers, and the like.
Generally, a feedback circuit is coupled to the power supply to regulate the DC output voltage. The feedback circuit includes a power transistor connected to the primary winding and a pulse width modulator for operating the power transistor. The pulse width modulator sends a pulse width modulated (PWM) signal to the gate of the power transistor to turn the power transistor on and off. As the power transistor is turned on and off, the energy stored in the primary winding is modulated to control the DC output voltage. The DC output voltage is used to change the duty cycle of the PWM signal. Thus, the DC output voltage provides feedback to the pulse width modulator for regulating the DC output voltage to a desired DC voltage level.
Additionally, the feedback circuit can operate a current sense circuit to sense the current through the primary winding of the transformer. The current sense circuit operates to prevent saturation of the primary winding of the transformer. If the current sense circuit determines that the sense current is greater than a threshold current level, the current sense circuit changes the duty cycle of the PWM signal to reduce the amount of current through the primary winding of the transformer. However, there are inherent propagation delays inside the current sense circuit and in turning off of the power transistor. Thus, by the time the current actually stops flowing through the power transistor, the current has increased above a desired threshold. In addition, a slope of the current is proportional to the input voltage. Therefore, the overshoot of the current above the desired threshold is higher at a high input voltage than at a low input voltage for the same propagation delay. As a result, the maximum power at the high input voltage is higher than at the low input voltage.
The use of the same reference symbols in different drawings indicates similar or identical items.